1. Field of the Invention
This invention relates to a wavelength division multiplexing optical transmission network having an optical ADM function and including dispersion compensators selected by a dispersion compensation method.
2. Description of the Related Art
Increase of the capacity of a network is proceeding together with the increase of the communication traffic. Recently, not only in backbone networks, but also in metro networks and access networks, construction of an optical transmission network based on the wavelength division multiplexing technique is demanded. In order to construct a network having a higher degree of flexibility, an optical OADM function capable of passing therethrough, adding or dropping light in a unit of a wavelength at a node.
FIG. 25 is a view showing an optical ADM apparatus (hereinafter referred to as node) The optical ADM apparatus can pass therethrough, drop or add a wavelength division multiplexed signal. Some optical ADM apparatus can add or drop only a particular wavelength (ch), and some other optical ADM apparatus can add or drop an arbitrary wavelength or wavelengths for each wavelength. As seen in FIG. 25, each node generally includes a dropping receiver 2 for receiving a drop signal from a wavelength division multiplexed signal and an adding transmitter 6 for adding to a wavelength division multiplexed signal.
Generally, since an optical signal is not a signal of a completely single wavelength, it suffers from waveform dispersion by wavelength dispersion (time delay difference depending upon the wavelength) in an optical transmission line. In order to suppress the waveform dispersion, it is necessary to suitably compensate for the dispersion in the transmission line or each apparatus. A popular dispersion compensation method is a method of inserting a dispersion compensator having a dispersion of the opposite sign to that of the dispersion generated in the transmission line to cancel the transmission line dispersion. Various dispersion compensators have been proposed including those which use a grating, those which use an optical interferometer and those which use an optical fiber.
FIG. 26 is a view illustrating an image of a residual dispersion and a dispersion tolerance. The axis of abscissa indicates the transmission point, and the axis of ordinate indicates the residual dispersion value. FIG. 26 illustrates a manner wherein a dispersion compensator is disposed at each of necessary places (four places in FIG. 26) of the transmission points to perform dispersion compensation. The residual dispersion (RD) is obtained by subtracting the dispersion compensation amount from a dispersion value which is accumulated as a signal propagates. The dispersion tolerance indicates an allowable range of the residual dispersion within which the receiver satisfies a particular characteristic. A solid line indicates the accumulated dispersion value of the signal on the long wavelength side in the wavelength division multiplexed signal, and a broken line indicates the accumulated dispersion value of the signal on the short wavelength side. FIG. 26 shows a signal having a compensated so that the residual dispersion value at the center wavelength between the long wavelength and the short wavelength may be the center of the dispersion tolerance.
On the other hand, in an optical transmission network, chirp is generated in a transmission line by a nonlinear effect appearing in the transmission line (self phase modulation (SPM) wherein the refractive index of the fiber relies upon the light intensity or cross phase modulation XPM wherein the refractive index is varied by the signal intensity of another wavelength). Also when a modulator is driven at a high speed, chirp which is a phenomenon that the wavelength of light varies transiently appears. If this variation is great, then deterioration in wavelength is caused by the wavelength dispersion characteristic of the optical fiber.
The chirp α is given by the following expression (1):α=(∂φ/∂t)/((1/2P)×∂P/∂t)  (1)where φ is the phase, P the optical power, and t the time.
Therefore, the target value of the dispersion tolerance or the residual dispersion strictly differs depending upon the number of spans and the span length. For example, the dispersion tolerance of a receiver varies depending upon the transmission rate, transmission distance, span number, fiber input power, dispersion compensator input power and so forth.
For example, the dispersion tolerance varies depending upon the chirp of the optical modulator like,                when α=−1, −100 to +800 ps/nm        when α=0, −600 to +600 ps/nm        when α=+1, −800 to +100 ps/nm        
FIG. 27 illustrates an example of the dispersion tolerance where the chirp is −1, 0 and 1.
It is to be noted, however, that the dispersion tolerance is that of a receiver when a modulation signal of the chirp α transmitted from a transmitter is not transmitted and hence does not undergo a nonlinear influence. It is to be noted that the value varies depending upon the characteristic and the transmission deterioration amount of a transmitter and a receiver. In this manner, the width of the dispersion tolerance and the absolute value of the width shift in response to the chirp.
Actually, also a dispersion value dispersion of the transmission line, a dispersion value dispersion of the dispersion compensator and so forth by the individual, temperature, secular change and so forth are involved, and also they are taken into consideration to decide whether or not the dispersion value falls within the dispersion tolerance.
FIG. 28 is a view illustrating a slope compensation rate which depends upon the specification characteristic of a dispersion compensator (DCF) constructed using a dispersion compensating fiber. When the slope compensation rate is 100%, also the residual dispersion value can be made constant over all channels of a band. On the other hand, where the slope compensation rate exceeds 100%, there is a tendency that the residual dispersion in those channels which are on the shorter wavelength side with respect to a channel of the center frequency (40 ch where the number of channels is 80 ch) increases. On the contrary, where the slope compensation rate becomes lower than 100%, the residual dispersion on the longer wavelength side with respect to the channel of the center frequency exhibits higher values. FIG. 26 illustrates the residual dispersion and the dispersion tolerance where the slope compensation rate is lower than 100%, and as a signal is transmitted, the residual dispersion value on the longer wavelength side exhibits an increase. Since it is generally difficult to achieve the slope compensation rate of 100%, the width between a maximum value and a minimum value of the residual dispersion value increases together with the transmission distance as seen in FIG. 26. As a factor other than the slope compensation rate, also, for example, the dispersion value dispersion of the transmission line or the dispersion compensator increases the width of the residual dispersion value. Accordingly, it is necessary to set the compensation amount of the dispersion compensator so that the target value of the residual dispersion may fall within a tolerance centered at an optimum value.
FIG. 29 is a view illustrating an example of path arrangement of a linear network constructed using an optical ADM apparatus. For simplified description, a model including four nodes is considered.
In the linear network,
a path group wherein a node A is a start point: A→B, A→C, A→D,
a path group wherein a node B is a start point: B→C, B→D,
a path group wherein a node C is a start point: C→D, and
a path group wherein a node D is a start point: none
are available. It is necessary to perform dispersion compensation design so that the dispersion tolerance may be satisfied between the transmission and reception sides in order that a predetermined error rate may be obtained in all of the path groups. It is to be noted that, if it is known in advance that not all paths are used, then dispersion design should be performed with regard only to the paths to be used.
FIG. 30 is a view illustrating an example of path arrangement of a ring network constructed using an optical ADM apparatus. For simplified description, a model including four nodes is considered.
In the ring network, since a path for going round the ring network is not used,
a path group wherein the node A is a start point: A→B, A→C, A→D,
a path group wherein the node B is a start point: B→C, B→D, B→A,
a path group wherein the node C is a start point: C→D, C→A, D→B, and
a path group wherein the node D is a start point: D→A, D→B, D→C,
are available (in FIG. 30, only those paths whose start point is the node A are shown). Thus, dispersion design should be performed with regard to all of the path groups mentioned similarly. Where a path for going round the ring network is used for testing or monitoring, paths
A→A, B→B, C→C, D→D
are required in addition to those mentioned above.
FIG. 31 is a flow chart illustrating a conventional dispersion compensation method.
FIG. 32 is a view illustrating dispersion compensation of a linear network by the conventional dispersion compensation method. For simplified description, a case of the linear network of full nodes wherein:                single mode fiber (SMF);        C-band (wavelength: 1,530 to 1,570 nm);        maximum wavelength number: 40 waves;        dispersion coefficient of ch1: 16 ps/nm/km,        dispersion compensating fiber coefficient: −77 ps/nm/km;        transmission line dispersion coefficient of ch20: 17 ps/nm/km, dispersion compensating fiber coefficient: −80 ps/nm/km;        transmission line dispersion coefficient of ch40: 18 ps/nm/km, dispersion compensating fiber coefficient: −83 ps/nm/km;        dispersion tolerance at each node: −100 to +800 (when the chirp is −1);        RD target at last node: center of dispersion tolerance or residual dispersion optimum value;        RD target at intermediate node: distributed in proportion to transmission line dispersion value; and        dispersion compensation pitch: Δ=50 ps/nm is considered. In the present example, a dispersion of dispersion values is not taken into consideration for the simplified description.        
It is assumed that, in the present linear network, a span #1 (route A, B) is 35 km; another span #2 (route B, C) is 5 km; and a further span #3 (route C, D) is 10 km.
A necessary dispersion compensation amount is calculated based on the center ch (ch20).
(1) At step S2, an average dispersion value of each span is calculated.Dispersion value of the span #1=17×35=595 [ps/nm]Dispersion value of the span #2=17×5=85 [ps/nm]Dispersion value of the span #3=17×10=170 [ps/nm]
(2) At step S4, it is decided whether or not the residual dispersion values (RD) of all of the routes satisfy the dispersion tolerance.
For example, since a route having a maximum dispersion is the route which passes the spans #1, #2 and #3, the accumulated dispersion value of the route is ch80=18×(35+5+10)=900 [ps/nm] and does not satisfy the dispersion tolerance.
(3) At step S6, a maximum dispersion route is detected.
The route having a maximum dispersion value is the route which passes the nodes A, B, C and D.
(4) At step S8, a residual dispersion target value is set.
For example, the center 350 ps/nm of the dispersion tolerance −100 to +800 ps/nm at the last node of the route which has the maximum residual dispersion value is set as the residual dispersion target value.
The residual dispersion optimum value is distributed in proportion to the transmission line dispersion value of each span. In particular:Residual dispersion optimum value of the span #1=350×(17×35)/(17×(35+5+10))=245 [ps/nm]Residual dispersion target value of the span #2=350×(17×5)/(17×(35+5+10))+245=280 [ps/nm]Residual dispersion target value of the span #3=350×(17×10)/(17×(35+5+10))+280=350 [ps/nm]At this time, the residual dispersion target values at the intermediate nodes satisfy the dispersion tolerance −100 to +800 ps/nm.
(5) At step S10, the accumulated residual dispersion value−residual dispersion target value is calculated as a dispersion compensation amount at each span. In particular:(Accumulated residual dispersion value−residual dispersion target value) at the span #1=17×35−245=350 [ps/nm]
A dispersion compensation amount of −350 ps/nm is requiredResidual dispersion amount after the dispersion compensation at ch20=17×35−350=245 [ps/nm]Residual dispersion amount after the dispersion compensation at ch40=18×35−350×83/80=279 [ps/nm](Accumulated residual dispersion value−residual dispersion target value) at the span #2=(245+17×5)−280=50 [ps/nm]
The dispersion compensation amount of −50 ps/nm is requiredResidual dispersion amount after the dispersion compensation at ch20=(245+17×5)−50=280 [ps/nm]Residual dispersion amount after the dispersion compensation at ch40=(279+18×5)−50×83/80=317 [ps/nm](Accumulated residual dispersion value−residual dispersion target value) at the span #3=(280+17×10)−350=100 [ps/nm]
The dispersion compensation amount of −100 ps/nm is required.Residual dispersion amount after the dispersion compensation at ch20=(280+17×10)−100=350 [ps/nm]Residual dispersion amount after the dispersion compensation at ch40=(317+18×10)−100×83/80=393 [ps/nm]
(6) At step S12, it is discriminated whether or not the residual dispersion values of all routes satisfy the dispersion tolerance. If the dispersion tolerance is satisfied, then the processing advances to step S14. However, if the dispersion tolerance is not satisfied by some of the residual dispersion values, then the processing advances to step S16. At step S14, the dispersion compensation amount selection is ended. At step S16, it is discriminated that selection of a dispersion compensation value is impossible.
Here, if the remaining dispersion value is calculated also with regard to the ch1 similarly as in (5) above, then since the residual dispersion values of all of the routes satisfy the dispersion tolerance, the dispersion compensation amount selection is ended.
From the foregoing, according to the conventional method, the dispersion compensation amounts of −350, −50 and −100 [ps/nm] are required for the spans #1, #2 and #3, respectively.
Now, dispersion compensation for a ring network is described. For simplified description, a ring network including totaling four nodes is considered. Same requisites as those of the linear network described hereinabove are used.
FIG. 33 is a view illustrating dispersion compensation of a ring network. The ring network includes a span #1 (route A, B) of 30 km, another span #2 (route B, C) of 28 km, a further span #3 (route C, D) of 25 km, and a still further span #4 (route D, A) of 5 km.
(1) First, an average dispersion value of each of the spans #1, #2, #3 and #4 is calculated.Dispersion value of the span #1=17×30=510 [ps/nm]Dispersion value of the span #2=17×28=476 [ps/nm]Dispersion value of the span #3=17×25=425 [ps/nm]Dispersion value of the span #4=17×5=85 [ps/nm]
(2) It is decided whether or not the residual dispersion values of all of the routes satisfy the dispersion tolerance.
For example, the accumulated dispersion value of the maximum dispersion route at ch40=18×(30+28+25+5)=1,584 [ps/nm], and this does not satisfy the dispersion tolerance.
(3) A maximum dispersion route is detected. The route having a maximum dispersion value is the route which passes the nodes A, B, C and D excepting the span #4 which is a minimum dispersion span in the ring.
(4) The center 350 ps/nm of the dispersion tolerance −100 to +800 ps/nm at the last node D of the maximum dispersion route is set as the residual dispersion optimum value. The residual dispersion optimum value is distributed in proportion to the transmission line dispersion value of each span. In particular:Residual dispersion target value at ch20 of the span #1=350×(17×30)/(17×(30+28+25))=127 [ps/nm]Residual dispersion target value at ch20 of the span #2=350×(17×28)/(17×(30+28+25))+127=245 [ps/nm]Residual dispersion target value at ch20 of the span #3=350×(17×25)/(17×(30+28+25))+245=350 [ps/nm]Residual dispersion target value at ch20 of the span #4=350×(17×5)/(17×(30+28+25))+350=371 [ps/nm]
(5) Then, the dispersion compensation amounts are determined from the accumulated residual dispersion value.(Accumulated residual dispersion value−residual dispersion target value) at the span #1=17×30−127=383 [ps/nm]
The dispersion compensation amount of −383 ps/nm is required (since the pitch Δ=−50 [ps/nm], −400 ps/nm nearest to −383 is used).Residual dispersion amount after the dispersion compensation at ch20=17×30−400=110 [ps/nm]Residual dispersion amount after the dispersion compensation at ch40=18×30−400×83/80=125 [ps/nm](Accumulated residual dispersion value−residual dispersion target value) at the span #2=(110+17×28)−245=341 [ps/nm]
The dispersion compensation amount of −341 ps/nm is required (−350 ps/nm nearest to −341 is used)Residual dispersion amount after the dispersion compensation at ch20=(110+17×28)−350=236 [ps/nm]Residual dispersion amount after the dispersion compensation at ch40=(125+18×28)−350×83/80=266 [ps/nm](Accumulated residual dispersion value−residual dispersion target value) at the span #3 =(236+17×25)−350=311 [ps/nm]
The dispersion compensation amount of −311 ps/nm is required (−300 ps/nm nearest to −311 is used).Residual dispersion amount after the dispersion compensation at ch20=(236+17×25)−300=361 [ps/nm]Residual dispersion amount after the dispersion compensation at ch40=(266+18×25)−300×83/80=405 [ps/nm](Accumulated residual dispersion value−residual dispersion target value) at the span #4 =(361+17×5)−371=75 [ps/nm]
The dispersion compensation amount of −75 ps/nm is required (−50 ps/nm nearest to −75 is used, although −100 ps/nm may be used, the lower value one is used).Residual dispersion amount after the dispersion compensation at ch20=(361+17×5)−50=396 [ps/nm]Residual dispersion amount after the dispersion compensation at ch40=(405+18×5)−50×83/80=443 [ps/nm]
(6) The accumulated residual dispersion values of all routes satisfy the dispersion tolerance.
From the foregoing, according to the conventional method, the dispersion compensation amounts of −400, −350, −300 and −50 [ps/nm] are required for the spans #1, #2, #3 and #4, respectively.
However, in the optical transmission system, since a target value for a residual dispersion value is determined and dispersion compensation is performed so that the residual dispersion value may approach the target value, a great dispersion compensation amount and a great number of dispersion compensators are required. That a great dispersion compensation amount is required gives rise to a disadvantage that the loss at the dispersion compensations increases as much, and in order to compensate for the loss, use of an amplifier having a high optical power is required. Further, that a great number of dispersion compensators are required gives rise to a problem that the an increased investment cost is required.